I want to know if strangers have accessed the database.
If so, I want to find out how they got access to the database.
I found a strange trigger in the database (screenshot below).
Your site contains a skimmer disguised as
googletagmanager.eu. Attackers managed to inject this skimmer in over 270 stores this week, and - sadly - also yours.
In the cases that I investigated, attackers used a flaw in MySQL/adminer to gain access to the victim sites. Because you shared an adminer screenshot, I suspect they used the same method on your store.
You should thoroughly scrutinize your site for hidden backdoors, disguised admin accounts and such. My eCommmerce anti-virus may help. See also:
@Jam: Your site is definitely infected. You can see the more details on below URL from Willem.
You can see more details on the the exact issue you have on his website.